We propose the study of two genetically defined loci in Drosophila melanogaster by characterizing their DNA coding regions isolated on recombinant DNA molecules. The first is the rosy lucus, which codes for the enzyme xanthine dehydrogenase; it has been genetically mapped better than any other gene in fruit flies. We hope to define precisely the transcribed and translated regions on the DNA sequence, to map the sites of various mutations on the DNA, and to determine the molecular consequences of the mutations. We will also examine variants which make abnormal levels of the enzyme; these may shed light on how the rosy gene is developmentally regulated. The second region of interest is the bithorax complex of homeotic genes. These genes control the developmental fate of two thoracic and all eight abdominal segments of the fly. Mutations in the complex cause transformations of one segment into another. We would like to map the mutation sites on the DNA, to find bithorax gene products, to learn when and where these products are made in development, and to see how their production is affected by various bithorax mutants. Eventually, we would like to discover how the bithorax genes turn on one developmental program and exclude all others.